dispatch_workqueue: check for bad syscall

Adding a check to verify that `syscltbyname` didn't fail. If it did, it
likely indicates that the `hw.activecpu` syscall doesn't exist.

If the call fails, return zero, indicating that we don't know how many
cpus are available.

Author: etcwilde

tests/dispatch_workqueue.c

@@ -55,7 +55,8 @@ activecpu(void)
         activecpu = si.dwNumberOfProcessors;
 #else
         size_t s = sizeof(activecpu);
-        sysctlbyname("hw.activecpu", &activecpu, &s, NULL, 0);
+        if (sysctlbyname("hw.activecpu", &activecpu, &s, NULL, 0) != 0)
+          return 0;
 #endif
 	return activecpu;
 }
@@ -66,6 +67,7 @@ int
 main(void)
 {
 	uint32_t ncpu = activecpu();
+	test_uint32_not("Failed to get CPU count", ncpu, 0);
 
 	dispatch_test_start("Dispatch workqueue");